The present invention is generally directed to the measurement of temperature, and more particularly to a novel method and apparatus for providing a three-dimensional indication of temperature variations within the interior of a body.
One field in which it is particularly desirable to be able to accurately measure temperature within the interior of a body is in the treatment of tumors by hyperthermia. Basically, this treatment approach involves the localized heating of the tumor to a specific temperature greater than normal body temperature. The localized heating is typically provided with a focused acoustic beam whose focal region is located in the tumor. During the treatment, it is necessary to monitor the temperature of the tumor tissue in order to be able to accurately control the frequency and/or intensity of the heating beam to assure that the tumor is being heated to the optimum temperature. Improper heating might result in either ineffectual treatment (not enough heat) or burning (too much heat).
In the past, the measurement of the temperature of the tumor tissue has been performed with invasive techniques, wherein a temperature sensitive device is inserted into the patient's body to be placed in direct physical contact with the tumor tissue. For example, the temperature sensing device could be a thermocouple attached to the end of a hypodermic needle. Other approaches might use fiber optics or similar such structures. The use of invasive temperature measuring techniques is undesirable in that the number of points at which temperature measurements can be made is limited due to the physical constraints on the number of invasive probes than can be practically utilized. In other words, only a sparse sampling of the temperature profile for the heated volume is available. In hyperthermia, as well as in other applications, it is important to be able to sample hundreds of individual points and obtain very fine resolution of the temperature profile, on the order of a few millimeters or less, for example. Other drawbacks associated with invasive techniques are that they require at least a minor incision and result in discomfort for the patient. In addition, they are limited in practical applications to the measurement of temperature in tumors located relatively close to the skin surface. They are not well suited for use in connection with deep-seated tumors or tumors that are located on or adjacent to sensitive vital organs.
Accordingly, it is a general object of the present invention to provide a novel method and apparatus for internally measuring the temperature of a body that does not require the use of invasive techniques. In this regard, it is an object of the invention to measure changes in the acoustic propagation properties of a body, to thereby provide an indication of temperature variations within the body.
The use of an acoustic signal to detect inhomogeneities or determine other internal properties of a body is generally known. For example, two similar acoustic inspection systems are disclosed in U.S. Pat. Nos. 3,233,450 and 3,771,355 issued to William J. Fry and Thomas D. Sachs, respectively. In the systems of these patents, a focused acoustic beam is used to excite, e.g. heat, the body to be measured in the area of interest. The excitation produces a temporary change in the acoustical characteristics in the region of the focal plane of the beam. A second acoustic beam, i.e. a sensing beam, is directed through the excited region of the body. The changes in acoustical characteristics, e.g. propagation velocity, are detected by the sensing beam. In the system of the Fry patent, the sensing beam detects the changes as the body is being excited. In the Sachs system, the acoustical characteristics are determined in the absence of any excitation and then again after the excitation has been applied.
Although the measurement of changes in acoustic properties has proven to be a viable noninvasive technique for determining internal characteristics of a body, heretofore known systems for practicing this technique are not without limitations. Foremost among these is the fact that the acoustic characteristic being detected, i.e. the velocity of the acoustic waves in the sensing beam, is averaged along the propagation path of the beam. The known systems do not have the capability to distinguish between changes occurring within the region of interest and changes outside of this region but within the sensing beam path. For example, where it is desired to monitor the heating of a tumor located within the liver of a person, the propagation velocity of the acoustic wave is going to be affected not only by the change in temperature of the tumor but also by movement of the liver caused by the action of the patient's breathing and the resulting motion of the diaphragm muscle, for example.
Due to this limitation, the prior systems are not well suited to scanning, for example to obtain a gradient profile of a particular area within the interior of a body. In other words, as the location of the beam is varied during scanning, its propagation path is going to change and thus any differences within the body but outside the area of interest will affect the detected result. Referring to the previous example, as the sensing beam is moved along the patient's abdomen to obtain a temperature profile of the liver tumor, the varying thickness of bones, muscle and other tissue that the beam must pass through will result in changes in the average velocity of the acoustic waves.
It is therefore another object of the present invention to provide a novel acoustic detection system for determining an internal characteristic of a body that is insensitive to variations occuring outside the region of interest.
It is a further object of the present invention to provide a novel acoustic detection system that examines acoustic characteristics of a body in a localized area and that can be scanned in three dimensions without being seriously degraded by variations outside the localized area.